Carbon nanotubes are crystalline carbon having a structure in which a thin layer of graphite crystal is rolled up to form a cylindrical shape. More specifically, carbon atoms forming carbon nanotubes form a graphene sheet having a flat or curved surface formed of six-membered rings of carbon molecules arranged in a hexagonal pattern, and the graphene sheet is rolled up to form a cylindrical carbon nanotube. In general, the diameter of carbon nanotubes is 0.4 nm to tens of nm, and the length of carbon nanotubes is normally several tens to several thousands or more times as long as the diameter thereof.
Such carbon nanotubes have a high degree of strength, excellent electrical conductivity, thermal conductivity, and lubrication property, and have therefore drawn attentions from various application aspects. Further, in order to allow carbon nanotubes to be distributed at lower costs, more efficient technologies for producing carbon nanotubes are being required.
A so-called gas-phase flow method has been known as one of technologies for producing carbon nanotubes. With this gas-phase flow method, carbon-containing materials and a catalyst metal are caused to flow along with carrier gas within a high-temperature furnace to thereby thermally decompose and synthesize materials such as carbon sources in the gas-phase, thereby producing carbon nanotubes. As this gas-phase flow method is suitable for mass production, there have been proposed a large number of improved technologies/processes for this method.
With this gas-phase flow method, there are cases in which a discharge pipe disposed downstream of a reaction tube is clogged with produced carbon nanotubes. Specifically, while carbon nanotubes synthesized in the reaction tube are supposed to flow in the downstream direction and to be finally discharged externally through the discharge pipe provided downstream of the reaction tube, there are cases in which, during this transfer process, the carbon nanotubes adhere to the inner surface of the discharge pipe. Once even a little amount of nanotubes is adhered to the inner surface of the discharge pipe, further carbon nanotubes are easily adhered to (or easily caught by) the carbon nanotubes already adhered to the inner surface, resulting in a rapid increase in the amount of carbon nanotubes accumulated in the discharge pipe. This may then finally result in clogging of the discharge pipe with the carbon nanotubes. In order to deal with this problem, conventionally, operation of the production apparatus is periodically interrupted and remove the carbon nanotubes adhered to the inner surface of the discharge pipe. However, the operations of periodical interruption of a production apparatus and removal of the carbon nanotubes as described above have caused problems including a reduction in the production efficiency of carbon nanotubes and an increase in the burden on operators.
JP 2001-73231 A and JP 2001-115342 A disclose technologies for preventing adhesion of carbon nanotubes onto the inner surface of the reaction tube. However, these technologies aim at prevention of adhesion of carbon nanotubes to a reaction tube and cannot therefore prevent adhesion to a discharge pipe. In addition, while a variety of other improved technologies have been proposed concerning the gas-phase flow method, there currently exist no technologies which can appropriately prevent carbon nanotubes from being adhered to and clogging a discharge pipe. Therefore, there has been difficult to increase the production efficiency of carbon nanotubes.